1. Field of the Invention
This invention relates to circuit breakers for protecting electric power circuits. More particularly, it relates to circuit breakers including a set of secondary contacts, which are controllable through an operator, such as a magnetically latchable solenoid. The invention also relates to circuit breakers providing an external status signal.
2. Background Information
Circuit breakers used in residential and light commercial applications are commonly referred to as miniature circuit breakers because of their limited size. Such circuit breakers typically have a pair of separable contacts opened and closed by a spring biased operating mechanism. A thermal-magnetic trip device actuates the operating mechanism to open the separable contacts in response to persistent overcurrent conditions and to short circuits.
Usually, circuit breakers of this type for multiple circuits within a residence or commercial structure are mounted together within a load center which may be located in a basement or other remote location. In some applications, it has been found convenient to use the circuit breakers for other purposes than just protection, for instance, for load shedding. It is desirable to be able to perform this function remotely, and even automatically, such as under the control of a computer.
U.S. Pat. Nos. 5,301,083 and 5,373,411 describe a remotely operated circuit breaker, which introduces a second pair of contacts in series with main separable contacts. The main contacts still interrupt the overcurrent, while the secondary contacts perform the discretionary switching operations. The secondary contacts are controlled by a solenoid, which is spring biased to close the contacts.
The solenoid has two coils, an opening coil and a holding coil. Initially, both coils are energized to open the contacts. Power to the opening coil is then turned off, and only the holding coil remains energized. Thus, continuous power is required to keep the main contacts open. When power to the holding relay is terminated, the spring recloses the secondary contacts.
U.S. Pat. No. 6,259,339 discloses a remotely operated circuit breaker, which introduces secondary contacts in series with main separable contacts. The secondary contacts are controlled by a solenoid, which has two coils, a first (or close) coil and a second (or open) coil. The coils are concentrically wound on a steel core supported by a steel frame. A plunger moves rectilinearly within the coils. A permanent magnet is seated between the steel core and the steel frame. When the close coil is energized, a magnetic field is produced which counteracts the magnetic field produced by the permanent magnet. A spring then pushes the contact arm closed. The secondary contacts are maintained in the closed state by a spring. When it is desired to open the secondary contacts, the open coil is energized which lifts the plunger to open the secondary contacts. With the plunger in the full upward position, it contacts the steel core and is retained in this second position by the permanent magnet. Subsequently, when the close coil is energized, the magnetic field generated is stronger than the field of the permanent magnet and therefore overrides the latter and moves the plunger back to the closed position.
It is known to provide an auxiliary switch in a circuit breaker in order to provide normally open and/or closed contacts for external status information. See, for example, U.S. Pat. Nos. 6,104,265; 5,552,755; and 5,264,673.
A controller circuit breaker 2, as shown in FIG. 1, consists of a series combination of circuit breaker main contacts 4 and contactor secondary contacts 6. If the main and secondary contacts 4,6 are both closed, then a line voltage 8 is supplied from a line terminal 10 to a load terminal 12. For example, such devices may be used for automatic lighting control. The contactor 14 of the controller circuit breaker 2 is bi-stable and is magnetically held opened by a magnet (not shown) and is mechanically held closed via a spring (not shown). This bi-stable action is important, since the contactor 14 must not change state should control power be lost. The transition from opened-to-closed or closed-to-opened is achieved magnetically by selectively energizing solenoid windings 16 or 18, respectively. The amp-turns of the close winding 16 oppose the permanent magnetic holding flux of the magnet, thereby allowing the spring to force and maintain the contactor 14 in the closed position. When the open winding 18 is energized, the amp-turns are reversed, which supports the permanent magnet's flux. This causes a solenoid plunger 20 to move to a location that allows the magnet to hold such plunger in a contactor-opened position.
Two problems exist with this system. First, the coils for the windings 16,18 must be small, in order to fit into the housing 22 of the circuit breaker 2 and, thus, cannot continuously support control voltage 24, shown as +24 VDC. Because of this, a form ‘C’ auxiliary contact 26 is used that follows the contactor 14. During the transition, for example, from closed-to-opened (the contact 26 is shown closed in FIG. 1), the control voltage 24 applied to the winding 18 is removed. The mechanical design of this system is very difficult, since the control voltage to the winding 18 cannot be removed before the contactor 14 changes state or the transition may cease. Also, the control voltage 24 must be removed from the winding 18 after the transition has occurred or the corresponding coil may burn up. Hence, there is a relatively narrow tolerance band that must be maintained for such a design to work.
Second, the circuit breaker 2 requires the status of the load terminal 12 (i.e., whether there is a load voltage). For there to be load voltage, both the main and secondary contacts 4,6 must be closed. This logical function is provided by passing the control voltage 24 through the auxiliary contact 26, which follows the contactor 14, and through an auxiliary switch 28, which follows the main contacts 4. However, the addition of the circuit breaker auxiliary switch 28 adds mechanical complexity and cost.
Accordingly, there is room for improvement in circuit breakers requiring external control and/or providing external status information.